I also use Foz Down and find it's very easy to dose accurately, once it's diluted.
I dilute mine in a 4 litre container and dose daily through a Profilux doser.
I measure phosphorus once a week and adjust the Foz Down dose amount. Very simple. I have no worries any more about overfeeding the tank.
Also, GFO removes trace elements from artificial sea water. It's difficult to tell when the GFO is exhausted and is messy to change.
Here is an article on the effects of GFO on trace elements.:
http://www.advancedaquarist.com/2012/2/chemistry
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